A 90μW 12MHz Relaxation Oscillator with a -162dB FOM

Abstract

Both ring oscillators and relaxation oscillators are subsets of RC oscillators featuring large tuning ranges and small areas. Figure 19.5.1 shows a typical relaxation oscillator with a capacitor and two switched current sources. Such relaxation oscillators have two advantages with respect to ring oscillators: 1) they have a constant frequency tuning gain; and 2) their phase can be read out continuously due to their triangular (or sawtooth) waveform. A major disadvantage of practical relaxation oscillators is their poor phase-noise compared to ring oscillators [1,2,4].The 1/f 2 phase-noise performance of oscillators can be compared using the FoM definition given in Fig. 19.5.3 [1]. Navid et al. have shown that at 290K thermodynamics limits the FoM of ring oscillators and relaxation oscillators to -165.3dB and -169.1dB, respectively [2]. Interestingly, they have also shown that the FoM of practical ring oscillators is generally better than about -160dB, while the FoM of practical relaxation oscillators is about 10dB worse. So in theory relaxation oscillators can be better, but in practice they are not. Part of the explanation is given in [2]; the noise added by the comparator, which is present in relaxation oscillators (cmp osc in Fig. 19.5.1) but not in ring oscillators, increases the phase-noise. We will show below that filtering this noise by exploiting a switched-capacitor discharge mechanism, the FoM of a practical relaxation oscillator can be as good as the FoM of ring oscillators. Fig. 19.5.1, I 1 charges capacitor C 1 . However, C 1 is not grounded, but connected across an OTA, and the discharge process exploits a switched capacitor, C 2 , which is reversed periodically. The operation of the circuit is described in the next sentences. has been subtracted from C 1 . V 3 = V + -V -is a sawtooth waveform. Subtracting this fixed charge packet filters out the noise of the oscillator comparator. The operation is illustrated in Fig. 19.5.3, which shows the control signal X, V 3 and the output of the comparator cmp out , V out , is also shown, which produces an edge whenever voltage V 3 reverses polarity. Suppose now that cmp osc is noisy and C 2 is reversed at t 4 instead of at t 3 . Although the duty cycle of V out is changed (at t 5 ), the active edge of V out at t 6 is unaffected and so is the phase-noise. This filter technique is similar to the anti-jitter circuit (AJC) technique used in open-loop jitter filters [3]; note that we apply a switched-capacitor circuit to subtract the charge packet, which is very power-efficient.Filtering out the noise of the oscillator comparator has two consequences: 1) the power dissipated by the oscillator comparator and its reference can be reduced without deteriorating the phasenoise; and 2) the two remaining contributions to the 1/f 2 phasenoise are the white noise of the charging and discharging mechanisms. It can be shown that the resulting FoM of such a relaxation oscillator is given by the equation in Fig. 19.5.3, where k is the Boltzmann constant, T the absolute tem...

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